1. Field of the Invention
The present invention relates to a multi-channel spectrograph, and, more particularly, to a spectrograph optimized to provide the largest possible number of independent spatial channels in the vertical plane and more modest spectral resolution in the horizontal plane.
2. Description of the Related Art
Spectrographs, and more recently scanning monochromators, have been in use for some time in an increasingly large number of applications. However, until quite recently, these instruments were limited to gathering and processing information through one channel. Light entered the instrument from a single source, and the instrument physically separated the light according to its wavelengths and presented as the output a single spectrum, most often dispersed in the horizontal plane.
In theory, nothing would have prevented the designers of early instruments, built around a prism as a dispersive element, to fashion a multichannel instrument, since they had good imaging properties due to their dioptric input and output optical systems working on axis. For each wavelength, the same point of the entrance slit was imaged as a different point in the image field. This presented the opportunity of using several spatially distinct sources of light at the input to obtain several distinguishable spectra in the image plane of a single instrument. However, in practice the modest sensitivity of early detectors as well as the small apertures (f/16 or less) of these early instruments forced designers to improve throughput at the cost of spatial resolution by introducing the concept of the entrance slit placed perpendicular to the axis of dispersion.
Later, when reflection gratings were introduced, allowing for easy extension into the UV and IR parts of the optical spectrum, dioptric optics were replaced by mirrors, which are easy to produce with broad band reflectivity. While dioptric optics work naturally on axis, mirrors are easier to use at an angle leading to very large astigmatic deformation of the image, an effect that becomes very important with fast instruments which require wide open beams and closely packaged elements.
An elegant approach to solve the astigmatic deformation of the image has been to ignore it by using the plane of the tangential focus as the image plane. In this configuration, a point of the object plane is transformed into a vertical line and a vertical slit into a slightly longer vertical image, which preserves spectral resolution. As a result, the instrument keeps a good spectral resolution at the cost of spatial resolution. This is of no consequence in applications where the only concern is measuring the spectral properties of a single sample. However, there are an ever growing listing of applications which would benefit from both spectral and spatial information.
The advent of two dimensional arrays of high quantum efficiency detectors, such as modern charge-coupled-device (CCD) and charge-induced-device (CID) 2-D detectors, and optical fibers to transport light has suggested the desirability of using spectrographs as multichannel dispersive systems capable of generating independent spectra of different sources. However, multispectra systems require a spectrograph capable of spectrally dispersing light along one axis while maintaining the spatial integrity of the input image vertically. In other words, the spectrum produced at one height at the focal plane of the spectrograph should be from one point at the corresponding height at the entrance slit.
The construction of such a spectrograph poses a challenge to designers. Conventional designs suffer from vignetting, astigmatism, coma, and other sources of crosstalk that destroy spatial purity of the resulting image at the focal plane. In recent years manufacturers have begun the introduction of high performance spectrographs allowing for some astigmatism correction and opening the field of multichannel spectroscopy. In 1989, CHROMEX Inc., of Albuquerque, New Mexico, introduced the FF-250/FF-500 family of fast (f/4) spectrographs, which use toroidal mirrors, instead of spherical mirrors, to correct astigmatism of the instrument. This advance allows the instruments to become multichannel instruments, particularly useful for multichannel applications while remaining capable of performing spectral measurements with the same resolution as their more conventional counterparts.
These improved instruments remain spectrographs primarily optimized for high spectral resolution in the horizontal direction. The astigmatic correction provided by toroidal mirrors allows for a limited number of independent spatial channels, probably more than enough for most applications, but cannot provide for high spatial resolution compatible with good imaging. This is the case because today fast instruments have by nature a high degree of astigmatism that can be corrected only in a narrow range of angles. Furthermore, the image field of these instruments has by design a high degree of curvature further limiting spatial resolution.
For an increasing number of new survey applications, where high spectral resolution is not usually needed, it is desirable to have a multichannel spectrograph which is optimized for the highest possible spatial resolution in the vertical plane and a more modest spectral resolution. Particularly important uses for such an instrument are in high resolution, remote sensing of earth resources, in infrared imaging, and in microscopy.